Many wireless communication systems employ beam forming transmission for conveying data to one or more wireless communication devices (WCD:s). As is well known in the art, beam forming transmission has the advantage that the used transmission power may be distinctly directed towards the WCD that the transmission is intended for, which increases the range and/or decreases the required transmission power, as well as decreases interference experienced by other WCD:s.
In wireless communication systems employing beam forming transmission, signals intended for many or all WCD:s associated with a network node (e.g. broadcast signals, control signals, etc.) may typically be transmitted using omnidirectional transmission, wide beam transmission, or beam sweep transmission; all of which are also well known in the art. Synchronization signals is one example of such signals intended for more than one WCD.
Problems with the approaches using wide beam transmission or omnidirectional transmission include one or more of: implicit reduction of the available maximum transmission power, increased hardware cost, and increased power consumption.
Legacy solutions for synchronization signal transmission typically use single port transmission solutions resulting in a wide beam transmission. This is since, according to legacy solutions, typically only one (or a few) antennas are implemented per antenna port in the dimension (e.g. the horizontal dimension) that determines the critical beam width and a narrow beam typically requires many antennas.
In more recent approaches where antenna arrays comprising multiple antenna elements in each dimension are used, the legacy methods for synchronization signal transmission may still be used in principle (invoking a small subset of the antenna elements in each dimension to mimic the wide beam of a legacy solution).
However, invoking only a few antenna elements in each dimension to create a wide beam may cause a reduction in the available transmission power. This is due to that each antenna element typically has a maximum power amplifier output limitation. Thus, using fewer antenna elements leads to a relatively lower maximum transmission power.
On the other hand, implementing antenna array power amplifiers with higher maximum output power capabilities typically entails increased hardware cost and increased power consumption.
Another approach to implementing full power wide beam synchronization signal transmission in beam forming systems is to implement a separate antenna element for this purpose in addition to the antenna array used for beam forming of dedicated transmission. Such a solution also entails increased hardware cost as well as increased size of the antenna installation.
Problems with the approaches using beam sweep transmission include one or more of: decreased coverage in one or more dimensions and inefficient use of transmission resources (e.g. time/frequency).
A typical implementation for antenna arrays comprising multiple antenna elements includes use of fast Fourier transform (FFT) beam forming, wherein multiple narrow beams (width depending on the number of antenna elements invoked for each beam) are swept in time and/or frequency to reach the entire area to be covered by the synchronization signal.
However, in more recent approaches (e.g. for the new radio, NR, concept) the number of beams allowed to be used for broadcast signal distribution may be restricted (e.g. since the number of possible beams may be very large in some emerging approaches). Under such restrictions, the aggregated angular coverage of a limited set of narrow FFT beams will typically not cover a desired area to be covered. On the other hand, if the FFT beams are widened by modification techniques known in the art (e.g. filtering) to achieve appropriate angular coverage, the maximum transmission power available with decrease as explained above; resulting in decreased radial coverage.
Therefore, there is a need for alternative approaches to synchronization signal transmission in wireless communication systems applying beam forming.